Carbon nanotubes hold great promise in many areas of science and technology, due to their unique physical properties and molecular-scale dimensions. A significant technological advance for these materials has been their incorporation as specific molecular transducers in nanosensors, molecular electronics and as molecular manipulation tools. This potential is based on the remarkable molecular recognition capabilities of carbon nanotubes through covalent chemical bonding, surface charge transfer or electrostatic changes when a specific molecule binds to a tube.
In order to achieve this specificity, nanotubes can be chemically, physically or biologically functionalized to recognize a particular target molecule and reject others in a complex environment. In addition, proof-of-principle demonstrations of nanotube functionalization for sensing or binding specific molecules in the gas and liquid phases have been successfully made.
The most commonly used geometry for nanotube based sensors is a chemically or biologically sensitive “field effect transistor”. The nanotube serves as a wire connecting lithographically defined source and drain metal electrodes on a doped silicon substrate having a thin insulating silicon oxide surface layer. Binding occurs over the length of the sides of the nanotubes in these devices. The electrically-conductive doped silicon serves as a backgate; noncovalent binding of target molecules is detected by changes in conductance of the device.
Recent advances in nanotube fabrication and AFM imaging with nanotube tips have demonstrated the potential of these tools to achieve high resolution images. Carbon nanotubes have been attached or grown on silicon AFM tips as high resolution AFM probes.
FIGS. 1A-B show a scanning electron micrographs of individual carbon nanotubes mounted to a silicon AFM probe tip by our team. The nanotube was picked up from a flat substrate supporting SWNTs grown by metal catalyzed chemical vapor deposition.
SWNTs are in many respects, ideal high resolution probe tips for AFM. SWNTs are single carbon atom thick hollow cylinders that are microns in length with diameters ranging from 0.7 to 5 nm. They can be used as high aspect ratio probes with radii comparable to molecular scale dimensions.
Carbon nanotubes are chemically and mechanically robust. They are the stiffest material known, with Young's moduli of about 1.2 Tpa, which limits the noise due to thermal vibrations and bending from degrading the ultimate obtainable resolution. Unlike other materials, carbon nanotubes buckle elastically under large loads, limiting damage to both the tips and the sample. Because SWNTs have well-defined molecular structures, interpreting AFM data becomes much easier since the tip-sample interaction is well characterized and reproducible.
As shown by Wong et al., “Covalently-Functionalized Single-Walled Carbon Nanotube Probe Tips for Chemical Force Microscopy”, J. Am. Chem. Soc. 120, 8557-8558 (1998), incorporated herein by reference for all purposes, SWNT AFM probe tips have been chemically functionalized uniquely at their very ends. This can be initiated by an electrical etching process, which is also used to shorten the attached SWNTs in order to achieve lengths suitable for high-resolution imaging. However, this approach still leaves the sides of the SWNT susceptible to non-specific binding of molecular species.
When SWNT tips are etched in an oxidizing environment (for example, in O2 ambient), the ends become functionalized with one or more carboxyl groups, based on bulk measurements carried out on chemically oxidized nanotubes. The tip can be chemically modified further by coupling organic amines to the carboxylate to form amide bonds. Alternatively, by etching the SWNT in a nitrogen environment, SWNT ends become functionalized with one or more amine groups, directly. The use of reactive amino chemistry is a common biochemical conjugation technique, and can be exploited further to take advantage of a wide range of chemical and biological means available for attaching fluorophores, antibodies, ligands, proteins or nucleic acids to the ends of the nanotubes with well-defined orientations.
The manipulation of a ligand-protein interaction with specific single molecules chemically and biologically coupled to the nanotube tip has been measured with AFM by Wong et al, “Covalently Functionalized Nanotubes as Nanometer Probes for Chemistry and Biology”, Nature, 394, 52-55 (1998), incorporated by reference herein for all purposes. However, nonspecific binding of molecules to the sidewalls of the nanotube is still frequent.
Often this is due to the hydrophobic nature of nanotubes. Hydrophobic sections of proteins or other biological molecules will bind to, and heavily coat, the nanotube sidewalls in a non-specific location. For example, in “Functionalization of Carbon Nanotubes for Biocompatibility and Bio-Molecular Recognition,” Nano Lett. 2, 285 (2002), incorporated by reference herein for all purposes, Shim et al. have shown that the protein streptavidin nonspecifically binds to as-grown SWNTs unless this nonspecific binding is prevented by coating the nanotubes with a surfactant, such as Triton, and poly(ethylene glycol), PEG.
Thus while known approaches have offered promise, improved techniques for employing carbon nanotubes for sensing and other functions are highly desirable.